Strain-rate sensitivity limit diagrams and plastic instabilities in a 6xxx series aluminum alloy Part I: Analysis of temporal stress–strain serrations

Abstract A method for the construction of “processing windows” to avoid negative strain-rate sensitivity and associated serrated flow in some aluminum alloys is described. The method is based on modern approaches and techniques of non-linear dynamics – linear stability analysis, amplitude Ginzburg–Landau equations, and bifurcation diagrams. The general instability area is identified as possessing a negative strain-rate sensitivity coefficient (SRS). The mathematical technique developed in the present paper was applied to a specific aluminum alloy, Al–0.4%Mg–0.2%Si, and yielded good results in terms of predicting the negative strain-rate sensitivity regions in the “strain rate–temperature” parameter space. It was clearly demonstrated that even though the instability area is located in the region of intermediate strain rates, a qualitative difference exists between the areas of (relatively) fast and (relatively) slow strain rates: while in the first case the dynamic behavior of the system is supercritical, in the second case it is subcritical. The second case is highly undesirable because it causes a sudden onset of stable stress serrations that are difficult to suppress (at a given temperature), while in the first case the development of instability is gradual and, consequently, more easily controllable.